In a device as known from PCT/EP94/00476 (WO 94/20402) the small diameter end region of the membrane is connected to the braking band such that the annular membrane extends outwardly from the large diameter edge of the braking band. When the annular membrane is displaced with its ring holder counter to the withdrawal direction of the yarn it undergoes a bending and pulling load from which the axial force load for the braking band results by which the braking band is pressed against the withdrawal surface. The annular membrane unifies in it the spring function for holding the braking band against the withdrawal surface and the function of a transmission body transmitting said axial force load from the ring holder to the braking band. In said arrangement, in practice, an irregular braking effect may occur for the yarn due to a misalignment between the braking band and the withdrawal surface along the essentially line-shaped contact zone between the braking band and the withdrawal surface. Moreover, the braking band is locally distorted counter to the withdrawal direction of the yarn in a yarn passage region when responding to the friction force and the repelling force of the yarn, said distortion hindering the achievement of a uniform tension level in the withdrawn yarn.
In other yarn feeding apparatuses (U.S. Pat. No. 5,409,043, U.S. Pat. No. 5,316,051), the axial force load is transmitted to the braking band by means of a hollow body having the shape of a frustocone into which the braking band is glued to the inner side of the hollow body. Said hollow body is supported at its outer side by means of a spring arrangement or an annular membrane which is elastic in the axial direction of the storage body, however, in the radial direction is relatively rigid. At the other side the hollow body is stiff in axial direction and elastic in the radial direction such that the flexible braking band is apt to deform locally in the yarn passage region and despite the axial force load. Said design requires multiple parts. The hollow body is highly loaded and tends to fatigue. The yarn feeding apparatus also consumes a large amount of space.
In another device known from DE-U-94 06 102, a plastic foam ring is glued to the outer side of the braking band which is held in the large diameter end of a rigid, frustoconical hollow body. A spring assembly engages at said hollow body which generates the axial force load for the braking band. Due to the relatively forceful spring assembly, the soft plastic foam ring is strongly deformed so that the inherent movability of the flexible braking band is restricted. The braking band bears onto a conical withdrawal surface of the storage body.
It is an object of the invention to create a device of the type disclosed above, as well as a yarn feeding apparatus in which along the contact zone between the braking band and the withdrawal surface, a uniform braking effect can be generated, and in which a substantially constant tension level can be maintained in the yarn being withdrawn.
This object can be achieved by providing a frusto-conical braking band oriented generally coaxially with the withdrawal surface defined on the storage body of a yarn feeding apparatus. An elastic annular membrane is connected at an outer end region thereof having the large diameter to a holding member, and an inner end region of the membrane having the small diameter is bent inwardly in a direction toward the withdrawal surface and is connected to the braking band.
The device saves space and consists of only two parts. Due to the bend of the small diameter end region of the annular membrane the braking band is no longer pulled against the withdrawal surface but is pushed against it. The braking band properly centers itself in operation thanks to the bend so that the braking effect along the contact zone remains uniform. It is imaginable that the braking band under the friction force and the repelling force of the yarn tends to locally enlarge the cone apex angle in the yarn passage region, i.e. to distort in withdrawal direction. A strong load exerted from the yarn onto the braking band, e.g. as occurs with a strong acceleration of the yarn or in case of high withdrawal speed, can result in a sickle-shaped gap between the withdrawal surface and the braking band in which gap the withdrawal resistance for the yarn may decrease. In such operational phases wherein the tension in the yarn being withdrawn is already increased, due to acceleration or high withdrawal speed, only a negligible contribution to the tension increase will be added by the braking device. To the contrary, in case of a low load exerted by the yarn on the braking band, the cone apex angle of the braking band in the yarn passage region will be reduced locally and thence the braking effect increases in order to raise the then already low yarn tension. This may help to maintain constant the yarn tension in the yarn being withdrawn. Due to this the device then operates with a self-compensation effect, i.e. the braking effect automatically is decreased when the yarn tension rises and automatically is increased when the yarn tension drops. With given outer dimensions of the ring holder mainly by the cross-sectional configuration of the annular membrane, an optimal large radial extension of the annular membrane can be used for its deformation. Since the connecting region is situated at an optimal small diameter the inherent movability or flexibility of the braking band is disturbed as little as possible, while the bend backwards and inwards (a bend like a U-turn) improves the self-centering capability of the braking band on the withdrawal surface.
The proper centering of the braking band and the constant tension level can be achieved by providing the membrane with a cross-section which, starting from the outer end region thereof, extends generally toward the braking band, and then extends to a position generally beyond same, and then extends inwardly toward the radial position of a small diameter edge of the braking band and finally extends, with its bend, to the braking band.
The major force load-transmission from the annular membrane to the braking band takes place within the diameter of the contact zone between the braking band and the withdrawal surface. Due to this, the braking band, preferably being a very thin flexible metal body, distorts itself in the yarn passage region in case of stronger yarn load by locally enlarging the cone apex angle in withdrawal direction. This helps maintain a constant yarn tension.
The transmission of the force load onto the braking band via the membrane is carried out very uniformly. The annular membrane is very flexible at the bent region thereof such that the inherent movability of the braking band due to the load from the yarn is hindered as little as possible. The arcuate bend also aids in the self-centering of the braking band on the withdrawal surface.
With regard to manufacturing, it is advantageous to leave both edges of the braking band free.
The self-centering effect of the braking band is enhanced by providing the membrane with an essentially S-shaped cross-section.
The wall thickness of the membrane, at least in the area of the small diameter end region thereof, decreases gradually so that the inherent movability or flexibility of the braking band is hindered only minimally.
The membrane, at the outer end region thereof, is bent in a direction opposite the bend of the inner end region. This outer bend is significantly deformed when transmitting the force load onto the braking band. The deformation motions and forces occurring in this region, however, are remote from the braking band so that they do not interfere with the proper operation of the braking band in its braking function.
The membrane, between the inner and outer bent portions has an arc-shaped interconnecting portion which separates or decouples the bent portions from one another.
In accordance with one embodiment, the decoupling of the inner and outer bent portions is enhanced by at least one further concentric undulation.
The bend of the annular membrane at the inner end region thereof having the small diameter may be fixed at the outer side and/or at the inner side of the braking band. Gluing or vulcanising lead to very homogenous and durable connection regions. In case that the bend is secured to the inner side of the braking band then the bend grips or folds around the small diameter edge of the braking band. Optionally, the connection is made directly at the small diameter edge of the braking band.
A cone apex angle of the braking band within a range of about 120.degree., e.g. between 90.degree. and 140.degree., allows the device to sensitively respond to accelerations or decelerations of the yarn. In connection with precise self-centering effect of the braking band at the withdrawal surface, a substantially constant tension level in the yarn being withdrawn can be achieved.
The withdrawal surface of the storage body is curved convexly, and an essentially line-shaped contact zone exists between the braking band and the withdrawal surface, in which contact zone the yarn is braked. A gradually tapering inlet leads to the braking nip which is advantageous for the yarn which during its withdrawal motion (balloon formation) also moves crosswise to its running direction. Moreover downstream of the braking nip, a gradually diverging outlet region is formed in which the outgoing yarn is able to move freely.
The interconnecting portion between the bent portions has a configuration which conforms to the withdrawal surface of the storage drum, which results in favorable kinematic relations. The interconnecting portion and the outwardly situated bend may fulfill an additional and desirable function, namely the function of a balloon limiter by which the yarn running with high withdrawal speed can be hindered from leaving the surface of the storage drum by too great a distance.
In a yarn feeding device equipped with the braking device according to the invention, a substantially constant tension level can be achieved in the yarn being withdrawn due to a proper self-centering of the braking band. The braking device consumes only mounting space inside the yarn feeding apparatus.